1. Field of the Invention
The invention relates to a process for the production of catalytic converters for purifying exhaust gases, and more particularly to a method for producing the catalytic converter comprising compressively closing the metal shell around the supporting mat-wrapped honeycomb substrate using an optimized mat density, resulting in a catalytic converter having a uniform and maximum safe compressive force exerted upon the honeycomb substrate.
2. Description of the Related Art
As is well known, the purification of exhaust gases from internal combustion engines, particularly in motor vehicles, is generally achieved by an exhaust gas purification system in which a ceramic element having a honeycomb cell structure acts as a catalyst carrier. More precisely, this honeycomb cell structure is covered with a catalyst which contains a precious metal which functions, in the presence of O.sub.2, to convert noxious components of the exhaust gas, such as HC and CO, to CO.sub.2 and H.sub.2 O. The honeycomb cell structure is housed within a gas-tight, sheet metal or cast-metal heat resistant housing or can.
Honeycomb structures currently employed are typically comprised of a ceramic material such as cordierite; a brittle material exhibiting limited mechanical strength. For this reason, catalytic converters in use today, typically include a supporting mat which is wrapped around the periphery of the honeycomb. This resilient material, which distributes any compressive forces on the ceramic, typically expands as the temperature increases. This being the case, the compressive supporting pressure on the honeycomb therefore increases at elevated temperatures, and in some degree compensates for the thermal expansion of the outer metal shell. Since the metal shell expands more than the enclosed ceramic honeycomb, this mat expansion with temperature rise prevents the honeycomb from becoming loose in the can shell.
There are known to the art various methods of fabricating catalytic converters as described above, one such method is described in U.S. Pat. No. 4,093,423 (Neumann) and U.S. Pat. No. 4,148,120 (Neumann), which generally discloses covering the outer circumferential surface of a monolithic carrier with an elastic intermediate layer and wrapping a rectangular piece of sheet metal around the covered body and thereafter pulling the ends of the metal shell together so as to exert a radial pressure on the surface of the carrier body within the range of 20 to 80 N/cm.sup.2.
Another such method of fabrication, commonly referred to as the "tourniquet wrap" method, is disclosed, for example in U.S. Pat No. 5,082,479 (Miller). This method involves forming a rectangular flat sheet metal piece into a cylindrical body having a lap joint. A mat-wrapped honeycomb is loosely inserted into the cylindrical metal can and the combined assembly is pulled together to form the desired mat compression. Thereafter, the lap joint is welded together thereby holding the can at the desired compression while at the same time preventing gas leakage It is known that the amount of compressive pressure exerted on a given honeycomb substrate by compressively closing the metal shell and supporting mat around the honeycomb substrate will be significantly affected by the honeycomb's outside diameter, the thickness and compliance of the supporting mat material and the metal shell dimensions. Each of these dimensions have manufacturing tolerances which must be carefully controlled to insure that adequate, but not excessive, radial pressure, is applied to the honeycomb substrate. Because of the great difficulty associated with holding very close manufacturing tolerances, a significant percentage of catalytic converters are either too tight, causing axial tensile loads on the honeycomb, or too loose resulting in unreliable axial retention of the honeycomb. Although the aforementioned prior art references each disclose a form of compressively closing the metal shell and the supporting mat in a manner which accounts for the tolerance problems, no suggestion exists to indicate precisely controlling the compressive closing so as to utilize an optimum closing force which ensures that there is sufficient retention while at the same time ensuring that the force utilized in closing is not so great as to damage the supporting mat material and the honeycomb substrate retained therein.